Propose pdflatex as default for preview

[Kornel Benko]

As it is now, we have 'latex' as default for our previews. But at least
opening UserGuide.lyx fails because
1.) call to dvipdf coredumpts
2.) calls to epstopdf fail (because underlined 'gs' coredumpts)

The reason is probably that the created lyxpreviewXXXXXX.dvi-file seems to contain
invalid entries, so the (from dvips) created eps files are not readable with gs.

The simple change in src/graphics/PreviewLoader.cpp cures the preview.
(Not changed the comments yet though)

	Kornel
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